In a disc player for reproducing an information signal from an optical disc, such as an optical digital audio disc, on which the information signal is recorded in the form of small pits arranged in a spiral track, a light beam is used for reading the information signal from the spiral track. The light beam is emitted from an optical head which is moved in the direction of the radius of the optical disc so as to scan the spiral track on the optical disc which is in a state of revolutions at a predetermined speed, and the light beam is required to trace correctly the spiral track and to be focused correctly on the surface of the optical disc on which the spiral track is formed. To make the light beam comply with these requirements, tracking servo-control and focus servo-control are performed. In the tracking servo-control, the position of a beam spot formed by the light beam on the optical disc in relation to the spiral track is detected to produce a tracking detection output and an optical element, such as a focusing lens, in the optical head or the optical head in its entirety is moved in the direction of the radius of the optical disc in response to the tracking detection output so as to cause the position of the beam slot to be placed correctly on the spiral track. In the focus servo-control, a focus condition of the light beam on the optical disc is detected to produce a focus detection output and the focusing lens in the optical head or the optical head in its entirety is moved in the direction of an axis of rotation of the optical disc in response to the focus detection output so as to cause the light beam to be focused correctly on the optical disc.
The tracking detection output is obtained in the form of a tracking error signal representing deviations of the beam spot on the optical disc from the center of the spiral track and the focus detection output is obtained in the form of a focus error signal representing defocus of the light beam on the optical disc. The tracking error signal and the focus error signal are usually derived from an information signal reproducing section together with a reproduced information signal. In the information signal reproducing section, a reading light beam, which comes from a portion of the optical disc on which the light beam emitted from the optical head is caused to impinge and modulated in intensity, is detected by a plurality of light detecting elements forming together a photodetector and detection output signals obtained from the light detecting elements in the photodetector are calculated at a signal processing circuit. Then, control signals are produced respectively based on the tracking error signal and the focus error signal which are derived from the information signal reproducing section containing the photodetector and the signal processing circuit, and supplied to driving means for moving the optical element such as the focusing lens in the optical head or the optical head in its entirety in the direction of the radius of the optical disc or the direction of the axis of rotation of the optical disc. As a result, the optical element such as the focusing lens in the optical head or the optical head in its entirety is moved in the direction of the radius of the optical disc or the direction of the axis of rotation of the optical disc in response to each of the tracking error signal and the focus error signal.
When the information signal is reproduced from the spiral track on the optical disc under the tracking servo-control and the focus servo-control performed as mentioned above in the disc player, and in the case where the optical disc has a defect where the surface or inside thereof is damaged or stained, the defect is read by the light beam emitted from the optical head to impinge thereon and the detection output signals obtained from the light detecting elements forming the photodetector are provided with variations caused in response to the defect of the optical disc. These variations in the detection output signal obtained from the light detecting elements bring about a missing portion of the reproduced information signal obtained from the signal processing circuit, which has such an extraordinarily low level as to be out of a predetermined range of the amplitude of the reproduced information signal, and further give rise to a relatively large noise in the form of a pulse in each of the tracking error and focus error signals derived from the signal processing circuit. That is, the defect of the optical disc results in the missing portion of the reproduced information signal and the relatively large noise in each of the tracking error and focus error signals.
The relatively large noise in the tracking error signal or the focus error signal resulting from the defect of the optical disc causes the control signal, which is supplied to the driving means for moving the optical element such as the focusing lens in the optical head or the optical head in its entirety in the direction of the radius of the optical disc or the direction of the axis of rotation of the optical disc, to take up an extraordinary level. Then, In the case where the control signal taking up such an extraordinary level is supplied to the driving means, the optical element such as the focusing lens in the optical head or the optical head in its entirety is moved abnormally so that the tracking servo-control or the focus servo-control is not properly carried out.
In the manner as mentioned above, an undesirable operation state wherein the tracking servo-control or the focus servo-control is not properly performed is easily caused in the disc player when the defect portion of the optical disc is read by the light beam emitted from the optical head.
To avoid such an undesirable operation state caused in the disc player, there has been proposed to provide a disc player with an arrangement for conducting a defect detection by which an operation state wherein the defect of the optical disc is read is detected in an information signal reproducing section including a photodetector and a signal processing circuit and for preventing both the tracking servo-control and the focus servo-control from being carried out abnormally when an output of the defect detection, namely, a defect detection output is obtained. In this arrangement proposed previously, the defect detection is carried out based on the reproduced information signal derived from the signal processing circuit in such a manner as mentioned below.
When the defect of the optical disc is read, a reproduced information signal Si comprises missing portions d each having a width corresponding to the width of the defect, as shown in FIG. 1A. Since the defect of the optical disc extends usually over several convolutions of the spiral track formed on the optical disc, the missing portions d occur periodically during a period of time depending on the size of the defect. The reproduced information signal Si containing the missing portions d is supplied to an envelope detector and a detection output signal Sv having a voltage level which corresponds to the envelope level of the reproduced information signal Si, as shown in FIG. 1B, is obtained from the envelope detector.
The detection output signal Sv has low level portions d' corresponding to the missing portions d contained in the reproduced information signal Si. Then, the level of the detection output signal Sv is compared with a reference voltage level Vo set as shown in FIG. 1B, and as a result, pulses Sp each taking up a high level in response to a portion of the detection output signal Sv which has a voltage level lower than the reference voltage level Vo, namely, the low level portion d' of the detection output signal Sv, as shown in FIG. 1C, are obtained.
In the case where pulses Sp thus obtained are used directly as the defect detection output successively from the first one thereof, false detection of the defect of the optical disc is apt to be caused. Accordingly, on the basis of the fact that the pulse Sp which result actually from the defect of the optical disc occurs for every revolution of the optical disc during a certain period of time in the same manner as the missing portion d in the reproduced information signal Si, the pulses Sp are supplied to an integration circuit having a predetermined integration time constant to be integrated in level and the defect detection output is destined to be made based on each of the pulses Sp which are obtained after an output level of the integration circuit reaches a predetermined voltage level.
A defect detecting part of the previously proposed arrangement which is operative to detect the operation state wherein the defect of the optical disc is read in such a manner as aforementioned comprises a circuit block including the integration circuit and produces the defect detection output after the defect of the optical disc is read and thereby the pulse Sp is obtained within a period of time of each one revolution of the optical disc. Therefore, the defect detection output derived from the defect detection part is delayed compared with a time point at which the defect of the optical disc is read actually. The time-lag arising thus on the defect detection output results in a problem that each of the tracking servo-control and the focus servo-control are not prevented effectively from being carried out abnormally in the operation state wherein the defect portion of the optical disc is read. Further, reliability in producing the defect detection output in response to the pulse Sp at the defect detection part is deteriorated due to the delay of the defect detection output.